MUSEUM OF COMPARATIVE ZOOLOGY. 91 



The advantages of vision in the new direction may have been due to 

 the more favorable relation of the cells to the direction of the newly ad- 

 mitted light as compared with that which came along the original course, 

 inasmuch as the latter was nearly perpendicular to the axes "of the retinal 

 cells (and therefore not favorable, upon Grenacher's theory, to the per- 

 ception of distinct images), whereas the former would be parallel to the 

 axes of some of the retinal cells, and therefore competent to furnish (upon 

 the development of the lens) a more distinct image. 



Any advantage of this nature Avould gradually lead to an extension of 

 the favorably located portion of the retina, and even to any modification 

 of the form of the layer as a whole whereby it should be brought into 

 still more favorable optical relations to the newly admitted light. This 

 might be accompanied by a gradual regressive modification of parts of the 

 retina not so situated as to be capable of profiting by light entering from 

 the new direction. In this way the originally symmetrical condition 

 would be replaced by conditions more and more unsymmetrical.* 



Thus in time a new lens might be formed and the old one atrophy ; 

 one region of the original retina might become converted into a new retina 

 with new bacilli at the deep ends of the cells, and the cells of the remaining 

 regions sink from their function of percipient elements to that of simple 

 pigment-cells. The disappearance of the original bacilli in the persist- 

 ently functional area of the original retina might be complete, or only 

 partial, 



A strong indication that the anterior median eye in Agelena previously 

 existed in the condition of a functional monostichous eye, the deep ends 

 of whose retinal cells were directly continuous with the optic-nerve fibres, 

 is found in the relation of the optic nerve to the present eye, and espe- 

 cially in its relation at diff"erent stages of its growth. Without some such 

 assumption the peculiar connection of the optic nerve with the retina 

 would remain apparently inexplicable ; but upon this assumption the 

 conditions appear as a natural consequence of the changes accompanying 

 involution. In the earliest stage in which the connection of the optic 

 nerve with the retina has been figured, before the appearance of the 

 bacilli (Figs. 1, 2), the nerve-fibres emerge from tlie outer and posterior 



* Grenadier has shown tliat there is an unsjTnnietrical condition of the retinal 

 cells and their bacilli in the anterior median eyes of Lycosa. (See Grenadier, 78, 

 Taf. III. Fig. 22 A, and text, p. 48.) This must doubtless be regarded as a secondary 

 differentiation, — i. c. as evolved after the infolding and from a more symmetrical 

 triplostichous condition ; but it is instructive as indicating the possibility of 

 regressive changes due to the altered functional requirements imposed on the retina. 



